The photoelectric monitoring of thickness variations of a fabric is known, for example, from German laid-open specification DE 30 43 852 A1, published July 1, 1982, according to which a textile web is transluminated by several elongate lamps extending across its width as it passes thereover from a supply roll to a take-up roll. Three focusing objectives lying in a row above the web path receive the emitted light by way of slot diaphragms and project images of the transluminated web portions upon three line-scanning photodiodes whose output signals are fed, via shift registers and preamplifiers, to inverting inputs of a pair of differential amplifiers with noninverting inputs having adjustable threshold voltages applied thereto. The system generates two trains of pulses whose width and relative spacing are determined by the separation of the consecutively scanned textile threads so that their integrated values are a measure of thread density. That system does not enable an automatic threshold adjustment on the recognition of thickness variations due to partial superposition of several fabric layers.